The use of a mechanism to vary the pitch of propeller blades on a propeller assembly is well known. This change of pitch provides the propeller blades with an optimum degree of blade angle for any particular flight condition.
A pitch change mechanism also allows for the blade pitch to be reversed to provide a braking effect, for example, when an aircraft is landing.
A typical such pitch change mechanism for use in a turbofan engine is disclosed in U.S. Pat. No. 3,922,852. In this mechanism, a linear actuator is interconnected with a helical cam means which in turn provides rotary motion of a single ring gear. The ring gear meshes with a bevel gear at the root of each fan blade to provide synchronised angular rotation to each of the fan blades.
The single ring gear acting on the fan blade shaft causes a large resultant axial force on the pitch change mechanism. This results in an axial load on the fan blade bearings, which in turn adds size and weight to the entire mechanism.